1. Field of the Invention
The present invention relates to a toner used for developers for developing electrostatic images in electrophotography, electrostatic recording, electrostatic printing, and the like, and also relates to a process cartridge and an image developing unit for electrophotography in which a developer containing the toner is used. More specifically, the present invention relates to a toner for electrophotography and a developer for electrophotography used for copiers, laser printers, plain paper facsimiles, and the like using a direct or indirect electrophotographic developing process, and also relates to a process cartridge and an electrophotographic image developing unit in which the developer for electrophotography is used. The present invention further relates to a toner and a developer used for full-color copiers, full color laser printers, and full color regular paper facsimiles, and the like each of which employs direct or indirect electrographic multi-color image developing process, also relates to a process cartridge and an image forming apparatus for electrophotography in which the developer for electrophotography is used.
2. Description of the Related Art
In electrophotographic apparatuses and electrophotographic recording apparatuses or the like, electric images or magnetic latent images have been developed into images through the use of toners. For example, in an electrophotographic process, a latent electrostatic image or latent image is formed on a photoconductor, and then the latent image is developed by using a toner to form a toner image. Typically, the toner image is transferred onto a transferring material such as paper and then fixed by means of heating or the like. A toner used for a latent electrostatic image is colored particles in which typically colorants, charge controlling agents, and other additives are included in a binder resin. There are two types of method for producing such a toner, namely, crushing method and suspension polymerization method. In the crushing method, colorants, charge controlling agents, anti-offset agents, and the like are melted and mixed to be uniformly dispersed in a thermoplastic resin, and the obtained composition is crushed and classified to thereby produce a toner. According to the crushing method, it is possible to produce a toner having excellent properties to some extent, however, there are limitations on selection of toner materials. For example, a composition produced by melting and mixing toner materials are required to be crushed and classified by using an economically available apparatus. To respond to the request, the melted and mixed component is forced to be made sufficiently brittle. For this reason, when the composition is actually crushed into particles, a wider range of particle size distribution is liable to be formed. When a copied image having excellent resolution and toner properties is expected to be obtained, for example, it surfers from the disadvantages that fine particles each having a particle diameter of 5 μm or less and fine particles each having a particle diameter of 20 μm or more must be eliminated by classifying the toner particles, and the yield is substantially low. In addition, in the crushing method, it is hard to uniformly disperse colorants and charge controlling agents, and the like in a thermoplastic resin. A dispersion liquid in which components are insufficiently dispersed negatively affects flowability of toner, developing property, quality of image, and the like.
In recent years, to overcome these problems involved in the crushing method, toner particles have been produced, for example, by suspension polymerization method (for example, see Japanese Patent Application Laid-Open (JP-A) No. 09-43909). However, toner particles obtained by suspension polymerization method are disadvantages in that such toner particles are poor in cleaning ability, although they are spherical. In developing and transferring an image having a low image area ratio, problems with cleaning failures may not occur because a residual toner remaining after transferring is a small amount, however, in developing and transferring an image having a high image area ratio such as a photographic image, further, a toner with which an untransferred image is formed due to a sheet-feeding failure or the like may occur as residual transferring toner on a photoconductor, causing background smear of image when such a residual transferring toner is accumulated. In addition, it causes smears on charge rollers or the like which contact-charges the photoconductor, which disenables exerting of intrinsic chargeability thereof.
For the above reason, a method for obtaining toner particles formed in indefinite shape by associating resin fine particles obtained by an emulsion polymerization method each other has been disclosed (for example, see Japanese Patent (JP-B) No. 2537503). However, in the toner particles obtained by the emulsion polymerization method, a large amount of surfactants remains not only on the surface of the toner particles but also in the inside of the toner particles even when they have been subjected to a washing treatment, which causes impaired environmental stability, a widen charge amount distribution, and image defective due to smears of the obtained images. There are problems that the remaining surfactants smear the photoconductor, charge rollers, developing rollers, or the like, which disenables exerting of intrinsic chargeability.
On the other hand, in a fixing step according to a contact-heat method in which fixing is performed by means of heating members such as a heat roller, releasing property of toner particles against the heating members, which is hereinafter referred to as anti-offset property, is required. Anti-offset property can be improved by making a releasing agent reside on surfaces of toner particles. In view of this tendency, a method has been disclosed in which anti-offset property is improved by making resin fine particles reside not only in toner particles but also reside on surfaces of the toner particles (for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2000-292973 and 2000-292978). However, this method involves a problem that the lower limit fixing temperature is raised, causing insufficient low-temperature fixing property, i.e. energy-saving fixing property.
In the method in which resin fine particles obtained by emulsion polymerization method are associated each other to thereby obtain a toner formed in indefinite shape, the following problems are caused. In other words, in the case where fine particles of a releasing agent are associated with toner particles in order to improve anti-offset property, the fine particles of the releasing agent are substantially taken into the toner particles, resulting in discouraging improvement in anti-offset property with sufficiency. Since resin fine particles, fine particles of releasing agents, fine particles of colorants or the like are fused and bound to toner particles randomly to thereby form the toner particles, variations arise in the composition or ratio of contents of the components between the obtained toner particles, and in molecular mass of the resin or the like, resulting in different surface properties between the toner particles, and disenabling of forming images steadily over a long period time. Further, in a low-temperature fixing system in which low-temperature fixing property is required, there has been a problem that fixing is inhibited due to resin fine particles which reside on surface of the toner, which disenables ensuring the range of fixing temperatures.
On the other hand, a new production method of a toner called the Emulsion-Aggregation method (EA method) is recently disclosed (for example, see Japanese Patent No. 3141783). In this method, toner particles are granulated from polymers which have been dissolved in an organic solvent or the like, contrary to the suspension polymerization method in which toner particles are formed from monomers. Japanese Patent No. 3141783 discloses some advantages of the emulsion-aggregation method in terms of an expansion of selection range of resins, controllability of polarity, and the like. In addition, it is advantageous in capability of controlling a toner structure, i.e. controlling a core-shell structure of toner particles. The shell structure comprises a layer containing only resins and is aims for reducing the amount of pigments and waxes exposed on surface of toner, and it is disclosed that the toner is not innovative in its surface condition and does not have an innovative structure (for example, see Characteristics of Toner Produced by New Production Method and the Prospects written by Takao Ishiyama and other two members, presented at The 4th-Joint Symposium—the Imaging Society of Japan and the Japan Society of Static Electricity). Thus, a toner produced by the emulsion-aggregation method is formed in a shell-structure, however, the toner surface comprises generally used resins and does not have an innovative structure, and there is a problem that when further lower-temperature fixing is pursued, it is not sufficient in heat resistant storage stability, and environmental charge stability.
In addition, in any of the suspension polymerization method, the emulsion polymerization method, and the emulsion aggregation method, styrene-acrylic resins are typically used, and with the use of polyester resins, it is hard to granulate toner and hard to control particle diameter, particle size distribution, and shape of toner. When further lower-temperature fixing is pursued, there are limitations in fixing property.
Further, aiming for excellent heat resistant storage stability and low-temperature fixing, the use of a polyester modified by urea-bonding has been known (for example, see Japanese Patent Application Laid-Open (JP-A) No. 11-133667), however, the surface of the toner is not particularly contrived, and there is a problem in environmental charge stability under strict conditions.
In the field of electrophotography, obtaining high-quality of image has been studied from various angles. Among these studies, it has been increasingly recognized that making toner in smaller diameter and in a spherical form is extremely effective in obtaining high-quality of image. There seems to be tendencies that with increasingly smaller diameter of toner, transferring property and fixing property are lowered, which leads to poor images. It has been known that transferring property is improved by forming a toner in a spherical shape (for example, see Japanese Patent Application Laid-Open (JP-A) No. 09-258474). In these circumstances, in the fields of color copiers and color printers, further higher-speed image forming is required. To respond to higher-speed image forming, an apparatus employing tandem-type technique is effectively used (for example, Japanese Patent Application Laid-Open (JP-A) No. 05-341617). The tandem-type technique is a technique by which images formed by image forming units are sequentially superimposed and transferred onto a single transferring paper sheet transported by a transferring belt to thereby obtain a full-color image on the transferring paper sheet. A color image forming apparatus based on the tandem-type technique has excellent characteristics of allowing a variety types of transferring paper sheet for use, having high-quality of full-color image, and enabling full-color images at high speeds. In particular, the characteristic which enables obtaining full-color images at high speeds is a characteristic unique to the tandem-type technique. The characteristic is not found in a color image forming apparatus employing other techniques. On the other hand, there have been attempts to achieve high-quality image as well as speeding-up using a toner formed in a spherical shape. To respond to further higher-speeding up, speedy fixing property is required, however, a spherically-shaped toner satisfying excellent fixing property as well as excellent low-temperature fixing property has not yet been realized so far.
In addition, when a toner is stored and delivered after production of the toner high-temperature and high humidity environment, low-temperature and low humidity environment are harsh conditions for the toner. A toner of which toner particles do not flocculate each other during the time of storage, has no degradation or exhibits less degradation in charge property, flowability, transferring property, and fixing property, and excels in storage stability has been required, an effective measure to respond to these requirements, particularly in spherically-shaped toners, has not yet been found so far.
In the mean time, a method is disclosed in which toner particles and inorganic powders such as various types of metallic oxides are mixed for use for the purpose of improving flowability and charge property of toner, and the mixed substance is called as external additives. There have been proposed a method in which treatments are performed with specific silane coupling agents, titanate coupling agents, silicone oil, organic acids, or the like for the purpose of modifying hydrophobic property and charge property of surfaces of the inorganic powders in accordance with the necessity, and a method for coating the inorganic powders with a specific resin. Examples of the inorganic powders known in the art include silicon dioxides (silicas), titanium dioxides (titanias), aluminum oxides, zinc oxides, magnesium oxides, cerium oxides, iron oxides, copper oxides, and tin oxides. Particularly, silica fine particles are frequently used which are obtained by reacting silica and/or titanium oxide fine particles with an organic silicon compound such as dimethyl dichloro silane, hexamethyl disilazan, and silicone oil, then substituting organic groups for silanol groups on surfaces of the silica fine particles and hydrophobizing the surfaces. However, these external additives are embedded in the toner base or liberated from the surface of toner due to mechanical stresses caused in inside of a developing unit with long-term use and when repeatedly used for a number of paper sheets, therefore, flowability and charge property of toner degrades. As a result, it may disenable obtaining proper image density and cause background smear. Thus, giving proper flowability and chargeability to toner base itself becomes an important issue.